Optical fiber waveguides have been used for communications systems. These optical waveguides are constructed of a transparent material such as glass. Waveguides comprise a center core surrounded by a cladding having an index of refraction less than the index of refraction of the core. Light propagates along the waveguide, being refracted within the core at its interface with the cladding. The theory of optical waveguides is well-known to those skilled in the art.
Long service life of optical waveguides requires high strength and durability. Chemicals, particularly water, may greatly affect the strength and durability of optical waveguides. Waveguides commonly comprise silicate glasses which are soluble in water. Surface cracks may appear and grow in silicate waveguides as the hydroxyl ion present in water attacks the silicon to oxygen bonds on the surface of the waveguide. These cracks result in deterioration of both the strength and the transmittance of the optical waveguide. When exposed to water or placed in an environment with a high moisture level, these cracks and the resulting deterioration of strength and transmittance grow with time. Thus, protection of the optical waveguide from contact with moisture and water is important. Various resin and plastic coatings have been used to protect optical waveguides from moisture.
When optical filament waveguides are used in wellbore logging operations, the severe environmental conditions encountered downhole greatly amplify the problems created by water. Further, resin and plastic coatings are permeable to water to some degree and are generally unsuitable for use in the high temperature, high pressure and highly corrosive downhole environment.
Extremely high pressures and temperatures may be encountered during wellbore logging applications. The solubility of silica in water increases as a function of temperature, pressure and pH. For example, at downhole pressures and temperatures of 15000 psi and 200.degree. C., the solubility of silicon dioxide in water is 1280 parts per million. The equilibrium vapor pressure of water at 200.degree. C. is 15.3 atmospheres. Optical filament waveguides comprising silica filaments are more susceptible to water and other chemical attack under these severe conditions of temperature and pressure.
Further, optical filament waveguides are generally used in stationary applications in most surface systems. When used in wellbore logging applications, the waveguides are subjected to extensive flexing in the logging cable. Movement of the waveguides and other cable components relative to one another also produces abrasion of the filaments. The abrasion and flexing produced by this movement also contribute to deterioration of both the durability and strength of the waveguide.
Resin and plastic coatings generally used for mechanical and chemical protection of waveguides for surface applications will not function properly in these severe conditions. Thus, it is desirable to provide an optical fiber waveguide with an impermeable coating which is resistant to moisture and other chemical attack and which will withstand the severe environmental conditions encountered in downhole operations. This protective coating should be particularly resistant to high pressures, to high temperatures and to the passage of water. This protective coating should also strengthen the optical filament.